Understanding Stepper Motors with DC Motor Theory
An Introduction to Stepper Motors
Part 1: Stepper Motor Theory and Design
Step motors (also known as Stepper motors) are DC motors that move in precise, discrete steps. They contain multiple coils that are organized into groups called phases. By energizing each phase in sequence, the motor will rotate, one step at a time. (Hence the term ‘step motor’.)
With a computer controlled ‘stepping’ you can achieve very specific positioning and speed control. For this reason, stepper motors are the motor of choice for many precision motion control applications.
The big appeal of step motors is the low cost, high reliability, high torque at low speeds and the simple, durable construction that operates in almost any environment.
During the next few posts, we will take a look at some of the basics of stepper motors and how they work for your project or business machinery.
In today’s post, let’s start with a brief overview of dc motor theory and design.
A step motor is a constant output power transducer. ‘Power’ is defined as torque multiplied by speed. This means motor torque is the inverse of motor speed.
To help better understand why a step motor’s power is independent of speed, we need to look how an ideal step motor functions.
An ideal step motor has zero mechanical friction with torque proportional to ampere-turns and the only electrical characteristic would be inductance.
‘Ampere-turns’ simply means that torque is proportional to the number of turns of wire in the motor’s stator multiplied by the current passing through those turns of wire.
‘Inductance’ describes the energy stored in a magnetic field anytime current passes through a coil of wire. Anytime there are turns of wire surrounding a magnetic material such as the iron in the motor’s stator, it will have an electrical property called inductance.
Inductance (L) has a property called inductive reactance, which may be thought of as a resistance proportional to frequency and therefore motor speed.
According to Ohm’s law, current is equal to voltage divided by resistance. In this case, we substitute inductive reactance for resistance in Ohm’s law and conclude motor current is the inverse of motor speed.
Since torque is proportional to ampere-turns (current times the number of turns of wire in the winding), and current is the inverse of speed, torque also has to be the inverse of speed.
In an ideal step motor, as speed approaches zero, its torque would approach infinity while at infinite speed torque would be zero. Because current is proportional to torque, motor current would be infinite at zero as well.
Electrically, a real motor differs from an ideal one primarily by having a non-zero winding resistance.
Also, the iron in the motor is subject to magnetic saturation, as well as having eddy current and hysteresis losses. Magnetic saturation sets a limit on current to torque proportionally while eddy current and hysteresis (iron losses) along with winding resistance (copper losses) cause motor heating.